Over the years, a variety of sensors have been developed for detecting and/or quantifying specific agents or compositions in a patient's blood, which enable patients and medical personnel to monitor physiological conditions within the patient's body. Illustratively, subjects may wish to monitor blood glucose levels in a subject's body on a continuing basis. Thus, glucose sensors have been developed for use in obtaining an indication of blood glucose levels in a diabetic patient. Such readings are useful in monitoring and/or adjusting a treatment regimen which typically includes the regular administration of insulin to the patient.
Presently, a patient can measure his/her blood glucose (BG) using a BG measurement device (i.e., glucose meter), such as a test strip meter, a continuous glucose measurement system (or a continuous glucose monitor), or a hospital hemacue. BG measurement devices use various methods to measure the BG level of a patient, such as a sample of the patient's blood, a sensor in contact with a bodily fluid, an optical sensor, an enzymatic sensor, or a fluorescent sensor. When the BG measurement device has generated a BG measurement, the measurement is displayed on the BG measurement device.
Current continuous glucose measurement systems include subcutaneous (or short-term) sensors and implantable (or long-term) sensors. Sensors have been applied in a telemetered characteristic monitor system. As described, e.g., in commonly-assigned U.S. Pat. No. 6,809,653, the entire contents of which are incorporated herein by reference, a telemetered system using an electrochemical sensor includes a remotely located data receiving device, a sensor for producing signals indicative of a characteristic of a user, and a transmitter device for processing signals received from the sensor and for wirelessly transmitting the processed signals to the remotely located data receiving device. The data receiving device may be a characteristic monitor, a data receiver that provides data to another device, an RF programmer, a medication delivery device (such as an infusion pump), or the like.
Regardless of whether the data receiving device (e.g., a glucose monitor), the transmitter device, and the sensor (e.g., a glucose sensor) communicate wirelessly or via an electrical wire connection, a characteristic monitoring system of the type described above is of practical use only after it has been calibrated based on the unique characteristics of the individual user. According to the current state of the art, the user is required to externally calibrate the sensor. More specifically, and in connection with the illustrative example of a diabetic patient, the latter is required to utilize a finger-stick blood glucose meter reading an average of two-four times per day for the duration that the characteristic monitor system is used. Each time, blood is drawn from the user's finger and analyzed by the blood glucose meter to provide a real-time blood sugar level for the user. The user then inputs this data into the glucose monitor as the user's current blood sugar level which is used to calibrate the glucose monitoring system.
Such external calibrations, however, are disadvantageous for various reasons. For example, blood glucose meters are not perfectly accurate and include inherent margins of error. Moreover, even if completely accurate, blood glucose meters are susceptible to improper use; for example, if the user has handled candy or other sugar-containing substance immediately prior to performing the finger stick, with some of the sugar sticking to the user's fingers, the blood sugar analysis will result in an inaccurate blood sugar level indication. Furthermore, there is a cost, not to mention pain and discomfort, associated with each application of the finger stick.
The current state of the art in continuous glucose monitoring (CGM) is largely adjunctive, meaning that the readings provided by a CGM device (including, e.g., an implantable or subcutaneous sensor) cannot be used without a reference value in order to make a clinical decision. The reference value, in turn, must be obtained from a finger stick using, e.g., a BG meter. The reference value is needed because there is a limited amount of information that is available from the sensor/sensing component. Specifically, the only pieces of information that are currently provided by the sensing component for processing are the raw sensor value (i.e., the sensor current or Isig) and the counter voltage. Therefore, during analysis, if it appears that the raw sensor signal is abnormal (e.g., if the signal is decreasing), the only way one can distinguish between a sensor failure and a physiological change within the user/patient (i.e., glucose level changing in the body) is by acquiring a reference glucose value via a finger stick. As is known, the reference finger stick is also used for calibrating the sensor.
The art has searched for ways to eliminate or, at the very least, minimize, the number of finger sticks that are necessary for calibration and for assessing sensor health. However, given the number and level of complexity of the multitude of sensor failure modes, no satisfactory solution has been found. At most, diagnostics have been developed that are based on either direct assessment of the Isig, or on comparison of multiple Isigs, e.g., from redundant and/or orthogonally redundant, sensors and/or electrodes. In either case, because the Isig tracks the level of glucose in the body, by definition, it is not analyte independent. As such, by itself, the Isig is not a reliable source of information for sensor diagnostics, nor is it a reliable predictor for continued sensor performance.
Another limitation that has existed in the art thus far has been the lack of sensor electronics that can not only run the sensor, but also perform real-time sensor and electrode diagnostics, and do so for redundant electrodes, all while managing the sensor's power supply. To be sure, the concept of electrode redundancy has been around for quite some time. However, up until now, there has been little to no success in using electrode redundancy not only for obtaining more than one reading at a time, but also for assessing the relative health of the redundant electrodes, the overall reliability of the sensor, and the frequency of the need, if at all, for calibration reference values.
The art has also searched for more accurate and reliable means for providing self-calibrating sensors, and for performing sensor diagnostics by developing a variety of circuit models. In such models, an attempt is generally made to correlate circuit elements to parameters that may be used in intelligent diagnostics, gross failure analysis, and real-time self-calibrations. However, most such models have had limited success thus far.
For each of the short-term sensors and the long-term sensors, a patient has to wait a certain amount of time in order for the continuous glucose sensor to stabilize and to provide accurate readings. In many continuous glucose sensors, the subject must wait three hours for the continuous glucose sensor to stabilize before any glucose measurements are utilized. This is an inconvenience for the patient and in some cases may cause the patient not to utilize a continuous glucose measurement system.
Further, when a glucose sensor is first inserted into a patient's skin or subcutaneous layer, the glucose sensor does not operate in a stable state. The electrical readings from the sensor, which represent the glucose level of the patient, vary over a wide range of readings. Thus, the sensor must first be stabilized. It is also desirable to allow electrodes of the sensor to be sufficiently “wetted” or hydrated before utilization of the electrodes of the sensor. If the electrodes of the sensor are not sufficiently hydrated, the result may be inaccurate readings of the patient's physiological condition. A user of current blood glucose sensors is instructed to not power up the sensors immediately. If they are utilized too early, current blood glucose sensors do not operate in an optimal or efficient fashion.